forward-swept wing

The Grumman X-29

The Grumman X-29 was an experimental aircraft that tested a forward-swept wing, canard control surfaces, and other novel aircraft technologies. The aerodynamic instability of this arrangement increased agility but required the use of computerized fly-by-wire control. Composite materials were used to control the aeroelastic divergent twisting experienced by forward-swept wings, also reducing the weight. Developed by Grumman, the X-29 first flew in 1984 and two X-29s were flight tested over the next decade.

The X-29A demonstrated high maneuvering and control in flight testing. A maximum angle of attack of 67° was reached. The configuration, combined with a center of gravity well aft of the aerodynamic center, made the craft inherently unstable. Stability was provided by the computerized flight control system

In a forward swept wing configuration, the aerodynamic lift produces a twisting force which rotates the wing leading edge upward. This results in a higher angle of attack, which increases lift, twisting the wing further. This aeroelastic divergence can quickly lead to structural failure. With conventional metallic construction, a torsionally very stiff wing would be required to resist twisting; stiffening the wing adds weight, which may make the design unfeasible.

The X-29 design made use of the anisotropic elastic coupling between bending and twisting of the carbon fiber composite material to address this aeroelastic effect. Rather than using a very stiff wing, which would carry a weight penalty even with the relatively light-weight composite, the X-29 used a laminate which produced coupling between bending and torsion. As lift increases, bending loads force the wing tips to bend upward. Torsion loads attempt to twist the wing to higher angles of attack, but the coupling resists the loads, twisting the leading edge downward reducing wing angle of attack and lift. With lift reduced, the loads are reduced and divergence is avoided

General characteristics

• Crew: one pilot
• Payload: 4,000 lb (1,810 kg)
• Length: 48 ft 1 in (14.7 m)
• Wingspan: 27 ft 2 in (8.29 m)
• Height: 14 ft 9 in (4.26 m)
• Wing area: 188.8 ft² (17.54 m²)
• Empty weight: 13,800 lb (6,260 kg)
• Max. takeoff weight: 17,800 lb (8,070 kg)
• Powerplant: 1 × General Electric F404 turbofan, 16,000 lbf (71.2 kN)

Performance

• Maximum speed: Mach 1.8 (1,100 mph, 1,770 km/h at 33,000 ft (10,000 m))
• Range: 350 mi (560 km)
• Service ceiling: 55,000 ft (16,800 m)

Sukhoi Su-47 Berkut

The Sukhoi Su-47 Berkut (Russian: Су-47 Беркут - Golden Eagle) (NATO reporting name Firkin) offered by Sukhoi in the early 1990s under the designation Su-37) during initial development, was an experimental supersonic jet fighter developed by Sukhoi Aviation Corporation. A distinguishing feature of the aircraft was its forward-swept wing that gave the aircraft excellent agility and maneuverability. While serial production of the type never materialized, the sole aircraft produced served as a technology demonstrator prototype for a number of advanced technologies later used in the 4.5 generation fighter SU-35BM and current 5th generation fighter prototype Sukhoi PAK FA.

The Su-47 is of similar dimensions to previous large Sukhoi fighters. Nonetheless, the aircraft includes reduced radar signature features, an internal weapons bay, and space set aside for an advanced radar. Though similar in overall concept to the Grumman X-29 research aircraft of the 1980s, the Su-47 is larger and far closer to an actual combat aircraft than the US design.

To solve the problem of wing-twisting, the Su-47 makes use of composite materials carefully-tailored to resist twisting while still allowing the wing to bend for improved aerodynamic behavior. Due to its comparatively large wingspan the Su-47 is to be equipped with folding wings in order to fit inside Russian hangars.

The Su-47 has extremely high agility at subsonic speeds, enabling the aircraft to alter its angle of attack and its flight path very quickly while retaining maneuverability in supersonic flight. The Su-47 has a maximum speed of Mach 1.6 at high altitudes and a 9g capability.

General characteristics

• Crew: 1
• Length: 22.6 m (74 ft 2 in)
• Wingspan: 15.16 m to 16.7 m (49 ft 9 in to 54 ft 9 in)
• Height: 6.3 m (20 ft 8 in)
• Wing area: 61.87 m² (666 ft²)
• Empty weight: 16,375 kg (36,100 lbs)
• Loaded weight: 25,000 kg (55,115 lb)
• Max. takeoff weight: 35,000 kg (77,162 lbs)
• Powerplant: 2 × Lyulka AL-37FU(planned) prototypes used 2 Aviadvigatel D-30F6 afterburning, thrust-vectoring (in PFU modification) turbofans
  • Dry thrust: 83.4 kN (18,700 lbf) each
  • Thrust with afterburner: 142.2 kN (32,000 lbf) each

• Thrust vectoring: ±20° at 30° per second in pitch and yaw

Performance

• Maximum speed: Mach 1.6 (Achieved in test flights ) (1,717 km/h, 1,066 mph)
  * At sea level: Mach 1.16 (1,400 km/h, 870 mph)
• Cruise speed: projected 1,800 km/h on dry thrust, 2650 km/h on full thrust
• Range: 3,300 km (2,050 mi)
• Service ceiling: 18,000 m (59,050 ft)
• Rate of climb: 233 m/s (46,200 ft/min)
• Wing loading: 360 kg/m² (79.4 lb/ft²)
• Thrust/weight: 1.16 (loaded) / 1.77 (empty)